History
Optical soundtracks for motion pictures were first demonstrated around the turn of the century, and since the 1930's have been the most common method of presenting sound with motion pictures. For analog optical soundtracks, the principle of operation is to first have an exciter lamp illuminate a narrow slit, which is perpendicular to the direction of film travel. A lens focuses the image of this slit onto the emulsion side of the film's analog soundtrack. This soundtrack runs parallel to the direction of film travel and lies between the picture and the sprocket holes. Located on the other side of the film, opposite the light source, a photocell or solar cell detects the amount of light passing through the film, and the current or voltage produced by the illuminated cell is amplified and ultimately sent to the theater's loudspeaker system. In modern systems, the transmission of light through the film is modulated by variations in soundtrack width, where an ideally transparent varying width of soundtrack is situated within an ideally opaque surrounding. This type of soundtrack is known as "variable area".
The first variable area soundtracks had one fixed edge, with the other edge a distance apart which varied with the required audio modulation. This type of optical soundtrack is referred to as "unilateral". The varying clear width causes the required modulation in light transmission received by the cell. It was realized in the late 1930's that error in light uniformity along the width of the slit results in the creation of distortion components. For example, a fall off in illumination at that end of the slit that corresponded to peak modulation level causes significant second harmonic distortion. In an effort to reduce this effect and other geometric distortion components, the "bilateral" variable area track was introduced. This format has two modulated edges, identical mirror images around a fixed centerline. This technique is immune to constant slope slit illumination error, but will develop distortion components under parabolic light error conditions.
A later development, which is now the standard monaural analog optical soundtrack format, is called "dual bilateral"(or "double bilateral" or "duo-bilateral") sound track. This format has two bilateral elements within the same soundtrack area, thus providing further immunity from illumination non-uniformity errors. In the mid 1970's Stereo Variable Area (SVA) tracks became increasingly popular, in which two independently modulated bilateral soundtracks are situated side by side in the same area as the normal monaural (mono) variable area track. FIG. 1 shows a portion of a representative 35mm motion picture film 2. The left total soundtrack is labeled "LT" and the right total soundtrack is labeled "RT". The film has sprocket holes 4 and picture areas 6. A two element solar cell is used in this case to provide two independent output signals, one derived from each of the two bilateral soundtracks.